HT83F20 datasheet(39/61 Pages) HOLTEK

39 / 61 page

HT83FXX

Rev. 1.00

May 12, 2009

Oscillator

Various oscillator options offer the user a wide range of

functions according to their various application require-

ments. Two types of system clocks can be selected

while various clock source options for the Watchdog

Timer are provided for maximum flexibility. All oscillator

options are selected through the configuration options.

The two methods of generating the system clock are:

· External crystal/resonator oscillator

· External RC oscillator

One of these two methods must be selected using the

configuration options.

More information regarding the oscillator is located in

Application Note HA0075E on the Holtek website.

External Crystal/Resonator Oscillator

The simple connection of a crystal across OSC1 and

OSC2 will create the necessary phase shift and feed-

back for oscillation, and will normally not require exter-

nal capacitors. However, for some crystals and most

resonator types, to ensure oscillation and accurate fre-

quency generation, it may be necessary to add two

small value external capacitors, C1 and C2. The exact

values of C1 and C2 should be selected in consultation

with the crystal or resonator manufacturer

¢s specifica-

tion. The external parallel feedback resistor, Rp, is nor-

mally not required but in some cases may be needed to

assist with oscillation start up.

Internal Ca, Cb, Rf Typical Values @ 5V, 25

°C

11~13pF

13~15pF

800k

Oscillator Internal Component Values

External RC Oscillator

Using the external system RC oscillator requires that a

resistor, with a value between 150k

W and 300kW,iscon-

nected between OSC1 and VSS. The generated system

clock divided by 4 will be provided on OSC2 as an out-

put which can be used for external synchronization pur-

poses. Note that as the OSC2 output is an NMOS

open-drain type, a pull high resistor should be con-

nected if it to be used to monitor the internal frequency.

Although this is a cost effective oscillator configuration,

the oscillation frequency can vary with VDD, tempera-

ture and process variations and is therefore not suitable

for applications where timing is critical or where accu-

rate oscillator frequencies are required. For the value of

the external resistor ROSC refer to the Holtek website for

typical RC Oscillator vs. Temperature and VDD charac-

teristics graphics. Note that it is the only microcontroller

internal circuitry together with the external resistor, that

determine the frequency of the oscillator. The external

capacitor shown on the diagram does not influence the

frequency of oscillation.

Watchdog Timer Oscillator

The WDT oscillator is a fully self-contained free running

on-chip RC oscillator with a typical period of 65

msat5V

requiring no external components. When the device en-

ters the Power Down Mode, the system clock will stop

running but the WDT oscillator continues to free-run and

to keep the watchdog active. However, to preserve

power in certain applications the WDT oscillator can be

disabled via a configuration option.

O S C 1

O S C 2

R f

T o i n t e r n a l

c i r c u i t s

I n t e r n a l

O s c i l l a t o r

C i r c u i t

C 1

C 2

C a

C b

R p

N o t e : 1 . R p i s n o r m a l l y n o t r e q u i r e d .

2 . A l t h o u g h n o t s h o w n O S C 1 / O S C 2 p i n s h a v e a p a r a s i t i c

c a p a c i t a n c e o f a r o u n d 7 p F .

Crystal/Resonator Oscillator

O S C 1

R O S C

O S C 2

f S Y S / 4 N M O S O p e n D r a i n

External RC Oscillator

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